High-strength erosion electrode

ABSTRACT

The invention concerns a high-strength erosion electrode having good electrical conductivity. The erosion electrode is made up of a steel core, an intermediate layer of copper or a copper-containing alloy, and an outer layer containing at least 40 % zinc. The steel core has a patented structure which contains between 0.6 and 1 wt. % carbon and occupies an area corresponding to between 50 and 75 % of the erosion electrode diameter, the intermediate layer occupying an area of between 5 and 40 % of the total diameter, the outer layer occupying an area of between 10 and 30 %, and the zinc content of the outer layer being between 40 and 60 wt. %.

FIELD OF THE INVENTION

The invention relates to a method for the manufacture of a wireelectrode for the spark-erosion process and a wire electrode for thismethod, which consists of a core of steel, an intermediate layer ofcopper or a high copper-containing alloy, and an outer layer with atleast 40% zinc.

BACKGROUND OF THE INVENTION

Wire electrodes, which are designed with multiple layers, which have acore of steel, an intermediate layer of copper arranged around the core,and an outer zinc-containing layer, are known, for example, from theDE-PS 29 06 245. All known wire electrodes, which are constructed with asteel core, do indeed have an increased strength compared with copper orbrass electrodes, however, all of these erosion electrodes have thedisadvantage that if they have comparatively high strengths, theirelectrical conductivity is very low and merely reaches 8 S·m/m². Thiscomparatively high tensile strength is particularly advantageous for thecutting of high or thick workpieces or, however, also for the cutting ofvery small parts since high wire tensions are here demanded. However,these known wire electrodes have the disadvantage that their erosionperformance is relatively low. In particular, in the case of cuttingvery small parts with erosion-wire diameters of up to 10 μm, high wirestrengths are demanded in order to reduce deflection and vibration ofthe wire to a minimum. Tungsten or molybdenum wires have been used up tonow for the cutting of very small parts with wire thicknesses of 100 μmand less, however, tungsten or molybdenum wires are extremely expensiveto manufacture. Erosion wires with a steel core and a brass outer layerhave been unable to be successful up to now for this purpose since theyalways, in comparison to tungsten wires, showed a lesser strength and apoorer erosion behaviour.

SUMMARY OF THE INVENTION

The basic purpose of the invention is to provide high-strength erosionelectrodes with a core of steel of the abovementioned type and a methodfor their manufacture, which electrodes have strengths which are 1800N/mm² and higher and, in addition, have a comparatively highconductivity of 10 S·m/mm² and more.

BRIEF DESCRIPTION OF THE DRAWING

The single drawing FIGURE illustrates cross-section of the inventivewire.

DETAILED DESCRIPTION

This purpose is attained by providing an erosion electrode with a coreof steel, an intermediate layer of copper or a high copper-containingalloy and an outer layer with at least 40% zinc, wherein the steel corehas a patented structure with a carbon content of 0.6 to 1 wt. %, whichoccupies an area of the core of 50 to 75% of the cross-sectional surfaceof the erosion electrode, wherein the intermediate layer has an area of5 to 40% and the outer layer has an area of 10 to 30%, and wherein thezinc content of the outer layer lies between 40 and 60 wt. %. Thepurpose is also attained by providing a method for the manufacture of ahigh-strength erosion electrode comprising a core of steel, anintermediate layer of copper or a high copper-containing alloy, and anouter layer of an alloy containing at least 40% zinc, wherein a steelwith a carbon content of 0.6 to 1 wt. % is used as the core, onto whichis applied the intermediate layer of copper or a high copper-containingalloy, wherein an outer layer of zinc or a high zinc-containing alloy isapplied onto the intermediate layer, wherein the wire, prior to a firstintermediate annealing, is subjected to a diffusion annealing such thaton the one hand the outer layer forms an alloy with a zinc content, themelting point of which is higher than the austenitization temperaturerequired for the patenting of the steel core, and on the other hand thediffusion process is carried out so long that, taking into considerationone or rather several following patentings of the erosion wire, theouter layer maintains the desired composition and strength, whereby theerosion electrode is patented during each intermediate annealing.

Thus, an erosion wire of the invention has a steel core K with a carboncontent of 0.6 to 1%, whereby a high copper-containing copper alloy isapplied as an intermediate layer ZS onto said steel core. Thisintermediate layer is covered by an outer layer MS which, when theerosion wire is finished, consists of a zinc alloy, the zinc content ofwhich lies between 40 and 60%. The condition for achieving thecomparatively high conductivity of the erosion electrode with an equallyhigh strength is that the core has a patented structure, consistsadvantageously of sorbite, whereby a further layer is arranged betweenthe outer layer and the core, which further layer contains high amountsof copper, that is, it is only slightly alloyed. Silver in the form ofthe alloy CuAgl has here proven to be particularly advantageous as thealloy element. Also a copper alloy with a small amount of zinc can beadvantageously utilized for the intended purpose. An outer layer hasbeen applied to this intermediate layer, which outer layer consists of azinc alloy, in which the zinc content reaches at least 40% and may reacha maximum of 60%.

A wire electrode of the invention could be manufactured in such a mannerthat a sleeve out of a brass pipe or brass band is applied to a steelcore enveloped with copper, and the composite is thereafter reduced bymeans of wire drawing. This type of manufacture becomes more problematicthe higher the zinc content in the outer layer is supposed to be. Afurther method could consist of providing a copper-enveloped steel wirewith a zinc-containing outer layer via a dipping method. The zinccontent could here be adjusted at random. The difficulty with thismethod is to apply the outer layer with an even thickness over thecircumference.

A very advantageous method for the manufacture of a wire electrode isthat a steel core is enveloped with a copper layer, whereby thestructure of the steel core should exist in a patented form. A zincalloy is thereafter then applied to this copper intermediate layer,whereby in the simpliest case the outer layer consists of pure zinc.This is followed by a diffusion annealing, the duration of which ischosen in dependency of the diffusion temperature such that the entireouter layer forms a zinc alloy, the zinc content of which amounts to 60%at a maximum. The duration of the diffusion annealing is furthermoredetermined such that the demanded areas for the core wire, theintermediate layer and the outer layer adjust, whereby these areas ofthe three sections are not only important for the strength achieved inthe final wire but also for its conductivity. A further decisive factorfor the strength, however and, also for the conductivity of the erosionwire is that in the final wire the core consists of a patentedstructure, whereby the electrical conductivity and strength are equallyoptimized.

The wire, the core of which should already exist in the patented stateis, after the diffusion annealing has occurred, reduced in its diameterby drawing.

The area of the core on the electrode should lie between 50 to 75% ofthe entire electrode. The area of the intermediate layer should liebetween 5 and 40% and the area of the outer layer between 10 to 30% . Ifone furthermore meets the demand that the zinc content of the outerlayer lies between 40 and 60% advantageously at slightly above 50% ,then one obtains an electrode with a conductivity which lies between 10and 18 S·m/mm², whereby at the same time strengths are obtained whichlie between 1800 and 2500 N/mm². It must hereby be noted that the demandfor high conductivity and high strength is in contrast, namely aninventive wire with a very high strength has a comparatively lowelectrical conductivity and vice versa. The wires of the invention canbe drawn to a diameter of below 10 μm, namely, to diameters which aresufficiently small for the up to now known uses. It has thereby beenfound to be particularly advantageous that these wires with a thindiameter can easily replace the up to now used tungsten and molybdenumwires and are by comparison significantly less expensive to manufacture,show no aging, have lower raw-material costs, and moreover offer ahigher safety in manufacture and have constant erosion characteristics.The achieved quality in manufacture is so great that the wire of theinvention will practically not break down on the erosion machine,whereas the breakdown of the up to now utilized molybdenum or tungstenwire amounts to up to 50% .

The high strength is particularly advantageous in the case of erosionwires with diameters of above 100 Am when workpieces with a greaterheight are to be cut since high wire tensions are required here in orderto keep bending and vibrations low.

An electrode of the invention is distinguished by a high conductivityand a comparatively high strength. Moreover, the cutting quality with ahigh exactness in contours is very good, whereby this precision isfavored by the high wire tension based on the high strength of the wire.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method ofmanufacturing a high-strength erosion electrode having a steel core, anintermediate layer of copper or a high copper content-containing alloyand an outer layer of an alloy containing at least 40% zinc, said methodcomprising the steps of: providing a steel core having a carbon contentof from 0.6 to 1 wt. %; applying an intermediate layer of copper or ahigh copper content-containing alloy onto the steel core; applying anouter layer of zinc or a high zinc content-containing alloy onto theintermediate layer to form a wire electrode; performing diffusionannealing on the wire electrode for a period of time sufficient to formthe outer layer into a zinc-containing alloy having a melting pointhigher than the austenitization temperature required for patenting ofthe steel core; and performing intermediate annealing on the wireelectrode under conditions sufficient to subject the wire electrode to apatenting treatment to form the high-strength erosion electrode.
 2. Themethod of claim 1, comprising the steps of: providing a patented steelwire with a copper intermediate layer as an initial wire; applying ahigh zinc content-containing alloy outer layer onto the intermediatelayer; performing diffusion annealing on the wire to form a brass outerlayer having a predetermined composition and strength; deforming thewire to reduce the diameter thereof; and performing a patenting stepafter each deformation step until the wire reaches its final diameter.3. An erosion electrode comprising a steel core, an intermediate layerof copper or a high copper content-containing alloy and an outer layercontaining from 40-60 wt. % zinc, wherein said steel core has a patentedstructure, a carbon content of 0.6 to 1 wt. % and occupies an area of 50to 75% of the cross-sectional area of the erosion electrode, theintermediate layer occupies an area of 5 to 40% of the cross-sectionalarea of the erosion electrode and the outer layer occupies an area of 10to 30% of the cross-sectional area of the erosion electrode.
 4. Themethod according to claim 2, wherein the outer layer after a firstdiffusion annealing step has a zinc content which does not exceed 60%.5. The method according to claim 4, wherein the initial wire has a corediameter of 0.8 mm and a layer thickness of the intermediate layer of0.1 mm, the outer layer applied onto the intermediate layer is 30 μm,the wire is then subjected to a diffusion annealing at 400° C. over 4hours and is thereafter drawn to a diameter of approximately 0.4 mm, theerosion wire is thereafter patented and is then drawn under, ifnecessary, further patentings to its final diameter.
 6. The erosionelectrode according to claim 3, wherein the conductivity of the erosionelectrode is at least 10 S·m/mm².
 7. The erosion electrode according toclaim 6, wherein the intermediate layer consists of copper with a smallamount of silver.
 8. The erosion electrode according to claim 7, whereinthe intermediate layer consists of CuAg1.
 9. The erosion electrodeaccording to claim 8, wherein the strength of the erosion wire is atleast 1800 N/mm².
 10. The method according to claim 1, wherein the outerlayer, after a first diffusion annealing step, has a zinc content whichdoes not exceed 60%.
 11. The method according to claim 2, wherein theinitial wire has a core diameter of 0.8 mm and a layer thickness of theintermediate layer of 0.1 mm, the outer layer applied onto theintermediate layer is 30 μm thick, the wire is then subjected to adiffusion annealing at 400° C. over 4 hours, drawn to a diameter ofapproximately 0.4 mm, thereafter patented and then drawn under, ifnecessary, further patentings to its final diameter.
 12. The erosionelectrode according to claim 6, wherein the strength of the erosion wireis at least 1800 N/mm².
 13. The erosion electrode according to claim 7,wherein the strength of the erosion wire is at least 1800 N/mm².
 14. Theerosion electrode according to claim 3, wherein the intermediate layerconsists of copper with a small amount of silver.
 15. The erosionelectrode according to claim 14, wherein the strength of the erosionwire is at least 1800 N/mm².
 16. The erosion electrode according toclaim 14, wherein the intermediate layer consists of CuAg1.
 17. Theerosion electrode according to claim 16, wherein the strength of theerosion wire is at least 1800 N/mm².
 18. The erosion electrode accordingto claim 3, wherein the strength of the erosion wire is at least 1800N/mm².